Use of lined passivation chamber in an iron powder passivating process



p 1, 1964 c. A. JOHNSON ETAL 3,147,106

USE OF LINED PASSIVATION CHAMBER IN AN IRON POWDER PASSIVATING PROCESSFiled April 6, 1961 0 MM &

IN VEN TOR S Clarence A. Johnson Wf/l/am Ko/lr United States Patent3,147,106 USE OF LINED PASSIVATION CHAMBER IN AN IRON POWDER PASSIVATINGPROCESS Clarence A. Johnson and William Volk, Princeton, NJ.,

assignors to Hydrocarbon Research, Inc., New York,

N.Y., a corporation of New Jersey Filed Apr. 6, 1961, Ser. No. 101,222Claims. (Cl. 75-.5)

This invention relates particularly to a process for the reduction ofpyrophoricity of iron powder, and the apparatus in which such ironpowders may be passivated.

In the Keith et al. Patent 2,900,246, a process is described for thereduction of iron oxide by the passage of hydrogen gas through a bed ofthe iron oxide at a rate to cause mobility or fluidization of the bed.This is normally carried out at about 750-950 F. and at a pressure of350600 p.s.i.g.

It has been found, however, that due to the purity and the fineness ofthe iron powder, which is normally such that all of it will go through a20 mesh screen, and at least 20% of which will go through 325 meshscreen, the powder is pyrophoric. In other words, on exposure to air atatmospheric temperatures, there is a tendency of the powder to reoxidizeand such reoxidation generates such heat that the mass may be convertedinto a combustible mixture unless it is adequately cooled, passivated,or retained in a non-oxidizing atmosphere.

It has been suggested that if the iron powder is heated to a temperaturein the order of 1200-1600 F. in the presence of an inert gas such asnitrogen for a period of time in the order of a half hour, the surfaceis so changed as to render the iron non-pyrophoric and suitable forpermanent storage, or use in powder metallurgy, or for other purposes.

It has been found, however, that at the temperature necessary to renderthe iron powder non-pyrophoric there is a tendency for a substantialamount of the powder to adhere to the passivation chamber, and this is aloss not only of material but in the cost of removing the agglomeratedparticles from the passivation chamber;

The object of the invention is to provide an improved form of apparatusand improved process steps for the passivation of finely dividednormally pyrophoric iron powder whereby such material is renderednonpyrophoric.

Further objects and advantages of the invention will appear from thefollowing description of a preferred form of embodiment thereof, and asmore particularly shown on the attached drawing which is illustrative ofthe invention, and in which FIGURE 1 is a schematic view of an iron orereduction apparatus and passivation chamber.

FIGURE 2 is a cross section taken along the line 2-2 of FIGURE 1.

FIGURE 3 is a substantially central vertical section, with parts inelevation, of a modified form of passivation chamber.

In accordance with the Keith et a1. patent hereinabove set forth, ironore may be reduced to a high purity iron powder by direct reduction,preferably in the presence of hydrogen. The ore in hopper is transferredby line 12 under the influence of a suitable gas such as hydrogen to thereducing reactor 14 which may have a plurality of separate ore beds 14a,14b, and 140. These beds are suitably interconnected by downcomers 16and control valves for the controlled downflow of the ore. A reducinggas such as hydrogen is introduced to the bottom of the reactor throughline 18 as described in the Stotler Patent 2,805,144 under suchconditions of temperature and velocity as to accomplish a fluidizedcontact with the ore. The oxygen removed from the ore 3,147,106 PatentedSept. '1, 1964 goes upward through the respective beds and thenceoverhead through line 20. The reduced ore is ultimately removed fromreactor 14 through line 22.

The iron powder is of the order of size in which all passes through a 20mesh Tyler screen with at least 50% passing through a 100 mesh screenand usually at least 20% will pass through a 325 mesh screen. The ironoxide is in the order of -98% reduced (oxygen removed) and is highlypyrophoric. In accordance with the invention, this pyrophoric ironpowder is discharged into a passivation chamber generally indicated at24 which may have an inert gas inlet at 26 and an outlet at 28. Nitrogengas is the preferable inert gas. Nitrogen gas or other inert gas ispassed through the passivator 24 to exclude any air or other oxidizinggas.

The passivation chamber 24 is suitably heated as for example by gasflames schematically indicated at 30 with the temperature controlled tothe desired extent. The gas inlet is at 31 and the products ofcombustion discharge at 32. In a particular operation the temperaturewas controlled within the range of l2001600 F. and preferably as near1400 F. as the controls would permit.

The passivation chamber 24 in FIGURE 1 is adapted to be inclined withrespect to the horizontal plane and rotated as a kiln through motor 33.The powder tends to flow to the lower portion of the passivator fromwhich it is continually removed as at 34. It is then cooled at 36 beforeit enters storage hopper 38. The cycle of passage of the are through thepassivation chamber is approximately thirty minutes.

Passivation is usually considered complete when the powder can be storedin moist air up to about 500 F., or it oxidizes at a rate of less than0.0007 gram of oxygen per minute/ grams in boiling water.

The liner or wall 40 for the passivation chamber as shown in FIGURE 2must be appropriate for the particular iron powder as magnetite powderreacts somewhat diflerently from a powder from ore which was primarilyhematite. In some cases reduced mill scale operated even differently.

The following table indicates the temperature conditions within thepassivation chamber 24 with respect to the different types of ore andthe type of lining and the effectiveness of such liners with respect toadhesion:

Hematite. Magnetite Mill Soale Hematite I do l 00 Graphite N0 adhesion.

Cerium Oxide (coat- Magnetite... 1,6001,700 F. Mill Sca1e 1,600l,700 FHematite. 2 000 F M agnetite- 1\/Iill Soale 1 60 o. Adhered. Noadhesion.

Alundum Stainless Steel Pyroceram in el Titanium Cast Iron PlatinumTantalum posed. No adhesion. Adhered (3 out of 4 samples). Zirconiumdecomposed. No adhesion. Adhesion. No adhesion. Adhesion.

Carborundnm Zirconium 1,6001,700 1,s0o-1,700 Magnetite 1,600 F MillScale 1,e00 F Zirconia (coating) Tungsten It will be apparent thatgraphite is entirely safe as a liner regardless of the ore treated.

Cerium oxide shows adhesion at 1800 F. but will be usually satisfactoryas normal passivation temperatures need not go so high.

Alundum which is a pure aluminum oxide (A1 is a border-line material andmust be restricted to ores that can be treated below 1600 P.

All other materials show some adhesion or material disintegration at1600 F. with some of the reduced ores.

Graphite is available in building blocks and other structural forms, andcan readily be fabricated into lining material for passivationequipment.

With a suitable liner such as graphite we avoid the need for expensivestainless steel vessels for a typical carbon steel vessel suitably linedwill have an unusually long life.

The linings may be applied in various manners as in the nature ofseparate brick or sheets or in some cases as for example zirconia,alundum or cerium oxide may be sprayed on the surface.

While the passivation chamber is shown in the nature of a kiln, it hasbeen found that passivation may be accomplished in a vertical chamber.This is more clearly shown in FIGURE 3 in which the passivation chamber42 is adapted to receive iron powder through th inlet 44 from which itwill drop down onto the screen 46. An inert gas entering at the bottomthrough line 48 will tend to fiuidize the powder, such gas beingdischarged at 50 Heat for the passivation is by means of hot inert gasessuch as flue gases that enter serpentine heat exchanger 52 at 54 anddischarge at 56. This tube may be of graphite or graphite coated andthus prevent any sticking or agglomeration of the iron powder and inaddition it will assure a uniform fluidization of the powder in thenecessary large scale passivation chamber, The passivation chamber mayalso be lined with a suitable lining as hereinbefore described.

While we have shown and described preferred forms of embodiment of ourinvention, we are aware that modifications may be made thereto which arewithin the scope and spirit thereof as set forth in the claims appendedhereinafter.

We claim:

1. The method of passivation of a finely divided pyrophoric iron powderreduced from metallic oxides from the group consisting of mill scale,hematite and magnetite which comprises the steps of flowing the reducedmaterial through a reaction zone, maintaining said zone at a temperatureof at least 1200 F. and not to exceed about 2000 F., flowing saidreduced material through the reaction zone in contact only with asurface material from the group consisting of graphite, cerium oxide,Alundum and Carborundum, and withdrawing said finely divided reducedmaterial from said zone as a powder after approximately thirty (30)minutes when its response to oxidation is such that it can be safelyhandled and stored in moist air at temperatures in the range of 400 F.to 500 F.

2. The method of passivation as claimed in claim 1 wherein the reducedmetallic oxide is from the group consisting of divided magnetite andfinely divided mill scale and the temperature of passivation in thereaction chamber is in the order of 1600 F. to 1700 F., and the surfacematerial is from the group consisting of graphite, cerium oxide,Carborundum and Alundum.

3. A passivation chamber for the passivation of a reduced ore from thegroup consisting of hematite, magnetite, and mill scale in a finelydivided and normally pyrophoric condition, said chamber having a reducedore inlet, a reduced ore outlet, and a wall between said inlet and saidoutlet, means to heat said ore to a temperature in the range of 1200 F.to 2000 F. to render said reduced ore non-pyrophoric, the wall of saidchamber being surfaced with a non-agglomerating coating from the groupconsisting of graphite, cerium oxide, Alundum and Carborundum, said oreheating means including a graphite heat exchange coil within saidchamber, and means to pass a heating gas through said coil.

4. A passivation chamber for the passivation of a reduced ore in afinely divided and normally pyrophoric condition, said chamber having areduced ore inlet, a reduced ore outlet and a wall between said inletand said outlet, means to heat said wall to a temperature in excess of1400 F. and below a temperature to fuse said reduced ore to render saidreduced ore non-pyrophoric, said wall being provided with anon-agglomerating coating of the class of graphite, cerium oxide,Alundum, and Carborundum.

5. A passivation chamber as claimed in claim 4 inclined with respect tothe horizontal plane, and means to rotate said chamber about the axis ofsaid inclination.

References Cited in the file of this patent UNITED STATES PATENTS1,452,627 Thornhill Apr. 24, 1923 2,603,561 Swann July 15, 19522,875,035 Graham et al Feb. 24, 1959

1. THE METHOD OF PASSIVATION OF A FINELY DIVIDED PYROPHORIC IRON POWDERREDUCED FROM METALLIC OXIDES FROM THE GROUP CONSISTING OF MILL SCALE,HEMATITE AND MAGNETITE WHICH COMPRISES THE STEPS OF FLOWING THE REDUCEDMATERIAL THROUGH A REACTION ZONE, MAINTAINING SAID ZONE AT A TEMPERATUREOF AT ELAST 1200*F. AND NOT TO EXCEED ABOUT 2000*F., FLOWING SAIDREDUCED MATERIAL THROUGH THE REACTION ZONE IN CONTACT ONLY WITH ASURFACE MATERIAL FROM THE GROUP CONSISTING OF GRAPHITE, CERIUM OXIDE,ALUNDUM AND CARBORUNDUM, AND WITHDRAWING SAID FINELY DIVIDED REDUCEDMATERIAL FROM SAID ZONE AS A POWDER AFTER APPROXIMATELY THIRTY (30)MINTUES WHEN ITS RESPONSE TO OXIDATION IS SUCH THAT IT CAN BE SAFELYHANDLED AND STORED IN MOIST AIR AT A TEMPERATURES IN THE RANGE OF 400F.TO 500*F.